1. Field Of The Invention
This invention relates to electrical induction apparatus, e.g. electric power transformers, specifically to the dielectric liquid coolants contained therein and especially to those coolants consisting of or containing as a constituent, polychlorinated biphenyl, PCB. More particularly, the present invention relates to methods for converting PCB-containing electrical induction apparatus, e.g. transformers, into substantially PCB-free transformers in order to qualify said transformers as "non-PCB" transformers under U.S. government regulations.
2. Prior Art
Because of their fire resistance, chemical and thermal stability, and good dielectric properties, PCB's have been found to be excellent transformer coolants. U.S. Pat. No. 2,582,200 discloses the use of PCB's alone or in admixture with compatible viscosity modifiers, e.g., trichlorobenzene, and such trichlorobenzene-PCB mixtures have been termed generically "askarels". These askarels may also contain minor quantities of additives such as ethyl silicate, epoxy compounds and other materials used as scavengers for halogen decomposition products which may result from potential electric arcing. ASTM D-2283-75 describes several types of askarels and delineates their physical and chemical specifications.
However, PCB's have been cited in the U.S. Toxic Substances Control Act of 1976 as an environmental and physiological hazard, and because of their high chemical stability, they are non-biodegradable. Hence, they will persist in the environment and are even subject to biological magnification (accumulation in higher orders of life through the food chain). Accordingly, in the United States, transformers are no longer made with PCB or askarel fluids. While older units containing PCB may still be used under some circumstances, it is necessary to provide special precautions such as containment dikes and maintain regular inspections. Transformers containing PCB's are at a further disadvantage since maintenance requiring the core to be detanked is prohibited, and the transformer owner remains responsible for all environmental contamination, including clean-up costs, due to leakage, tank rupture, or other spillage of PCB's, or due to toxic by-product emissions resulting from fires. To replace a PCB-containing transformer, it is necessary to (1) remove the transformer from service, (2) drain the PCB and flush the unit in a prescribed manner, (3) remove the unit and replace with a new transformer, and (4) transport the old transformer to an approved landfill for burial (or to a solid waste incinerator). Even then, the owner who contracts to have it buried still owns the transformer and is still responsible (liable) for any future pollution problems caused by it. Liquid wastes generated during replacement must be incinerated at special approved sites. Thus replacement of a PCB transformer can be expensive, but more importantly, since most pure PCB or askarel transformers are indoors, in building basements or in special enclosed vaults with limited access, it may not be physically feasible to remove or install a transformer, nor would it be desirable from an asset management perspective.
A desired approach to the problem would be to replace the PCB oil with an innocuous, compatible fluid. A number of fluid types have been used in new transformers as reported in Robert A. Westin, "Assessment of the Use of Selected Replacement Fluids for PCB's in Electrical Equipment", EPA, NTIS, PB-296377, Mar. 1, 1979; J. Reason and W. Bloomquist, "PCB Replacements: Where the Transformer Industry Stands Now", Power, October, 1979, p. 64-65; Harry R. Sheppard, "PCB Replacement in Transformers", Proc. of the Am. Power Conf., 1977, pp. 1062-68; Chem. Week, 130, 3, 24 (1/20/82); A. Kaufman, Chem. Week, 130, 9, 5 (3/3/82); CMR Chem. Bus., Oct. 20, 1980, p. 26; Chem. Eng., July 18, 1977, p. 57; Belgian Pat. No. 893,389; Europ. Plastic News June, 1978, p. 56. Among these are silicone oils, e.g., polydimethylsiloxane oils, modified hydrocarbons (for high flash points, e.g. RTEmp, a proprietary fluid of RTE Corp.), synthetic hydrocarbons (poly-alpha-olefins), high viscosity esters, (e.g. dioctyl phthalate and PAO-13-C, a proprietary fluid of Uniroyal Corp.), and phosphate esters. A number of halogenated alkyl and aryl compounds have been used. Among them are the liquid trichloro- and tetrachlorobenzenes and toluenes and proprietary mixtures thereof (e.g., liquid mixtures of tetrachlorodiarylmethane with trichlorotoluene isomers). Liquid mixtures of the trichloro- and tetrachlorobenzene isomers are particularly suitable because of their low flammabilities (e.g., high fire points) and similar physical and chemical properties to askarels being removed. Other proposed fluids are tetrachloroethylene (e.g. Diamond Shamrock's Perclene TG) and polyols and other esters.
Of all the non-PCB fluids, silicone oils have been the most widely accepted. Their chemical, physical, and electrical properties are excellent. They have high fire points (&gt;300.degree. C.), and no known toxic or environmental problems These oils are trimethylsilyl end-blocked poly(dimethylsiloxanes) of the formula: EQU (CH.sub.3).sub.3 SiO[(CH.sub.3).sub.2 SiO].sub.n Si(CH.sub.3).sub.3
wherein n is of a value sufficient to provide the desired viscosity (e.g., a viscosity at 25.degree. C. of about 50 centistokes). Commercial silicone oils suitable for use are available from Union Carbide (L-305), and others. In addition, U.S. Pat. No. 4,146,491, British Pat. Nos. 1,540,138 and 1,589,433 disclose mixtures of silicone oils with a variety of additives to improve electrical performance in capacitors, transformers and similar electrical equipment, and disclose polysiloxanes with alkyl and aryl groups other than methyl.
Replacement of PCB-containing askarels in older transformers with silicone oils or one of the other substitute fluids would seem to be a simple matter, but it is not. A typical transformer contains a great deal of cellulosic insulating material to prevent electrical coils, etc., from improper contact and electrical arcing. This material is naturally soaked with askarel, and may contain from 3 to 12% of the total fluid volume of the transformer. This absorbed askarel will not drain out, nor can it be flushed out by any known means, however efficient. Once the original bulk askarel is replaced with a fresh non-PCB fluid, the slow process of diffusion permits the old absorbed askarel to gradually leach out, and the PCB content of the new fluid will rise. Thus, the new coolant becomes contaminated.
For purposes of classification of transformers, the U.S. government regulation has designated those fluids with greater than 500 ppm PCB as "PCB transformers", those with 50-500 ppm PCB as "PCB contaminated transformers", and those with less than 50 ppm PCB as "non-PCB transformers". While major expenses may be entailed with the first two classifications in the event of a spill or the necessity of disposal, the last category is free of U.S. government regulation. To achieve the last classification, the PCB concentration must remain below 50 ppm for at least 90 days, with the transformer in service and sufficiently energized that temperatures of 50.degree. C. or higher are realized. This requires a 90-day averaged rate of elution of 0.56 ppm/day. It is anticipated that most, if not all, states of the United States will adopt regulations which may be the same as, or stricter, than U.S. government regulations. More lenient regulations may be possible elsewhere.
There are a number of commercial retrofill procedures on the market including those described in "The RetroSil PCB Removal System", Promotional literature of Dow Corning Corp., #10-205-82 (1982), and trade literature of Positive Technologies, Inc. on the Zero/PC/Forty process. These utilize initial clean-out procedures of as high efficiency a possible during which the electrical apparatus is not in operation. Most include a series of flushes with liquids such as fuel oil, ethylene glycol, or a number of chlorinated aliphatic or aromatic compounds. Trichloroethylene is a favorite flush fluid. Some processes, such as the Positive Technologies, Inc. Zero/PC/Forty process use a fluorocarbon vapor scrub alternating with the liquid flushes. When the initial clean-out procedure is complete, the transformer is filled with silicone fluid. As effective as these elaborate flushing procedures might have been expected to be, they cannot remove PCB adsorbed into the interstices of the cellulosic material. Consequently, the PCB content of the silicone coolant gradually rises as the residual PCB leaches out while the transformer is in use. Therefore, if one wishes to reach a PCB-free state ("non-PCB" as defined by U.S. government regulations), it is necessary to either periodically change-out, or continually clean up, the silicone fluid until a leach rate of less than 50 ppm for 90 days is reached.
Periodic change-out is very expensive, and because both the silicone and PCB are essentially non-volatile, distillation to separate them is not practicable and other methods of separation are expensive or ineffective. Dow Corning in its RetroSil process uses a continual carbon filtration to clean up the fluid ("The RetroSil PCB Removal System", Promotional literature of Dow Corning Corp., #10-205-82 (1982); Jacqueline Cox, "Silicone Transformer Fluid from Dow Reduces PCB Levels to EPA Standards", Paper Trade Journal, Sept. 30, 1982; T. O'Neil and J. J. Kelly, "Silicone Retrofill of Askarel Transformers", Proc. Elec./Electron. Insul. Conf., 13, 167-170 (1977); W. C. Page and T. Michaud, "Development of Methods to Retrofill Transformers with Silicone Transformer Liquid", Proc. Elec./Electron. Insul. Conf., 13, 159-166 (1977)). Westinghouse in U.S. Pat. No. 4,124,834 has patented a transformer with a filtration process for removing PCB from the coolant, while RTE in European Pat. No. 0023111 has described the use of chlorinated polymers as an adsorbing media. However, the filters used in these processes are very expensive and the removal of PCB is very ineffective, due both to lack of selectivity and the very low concentrations of PCB being filtered. In lieu of filtration, procedures have been proposed involving decantation (U.S. Pat. No. 4,299,704) which is impractical due to solubility limitations, and only good at high concentrations; extraction with polyglycols (F. J. Iaconianni, A. J. Saggiomo and S. W. Osborn, "PCB Removal from Transformer Oil", EPRI PCB Seminar, Dallas, Tex., Dec. 3, 1981) or with supercritical CO.sub.2 (Richard P. deFilippi, "CO.sub.2 as a Solvent: Application to Fats, Oils and Other Materials", Chem. and Ind., June 19, 1982, pp. 390-94), and chemical destruction of the PCB's with sodium (British Pat. No. 2,063,908). None of these schemes have been found to be economically or commercially practical for askarel transformers. However, the filtration scheme could be a reasonably effective, though expensive, procedure if it were not for the fact that the leach rate is so slow that it could take many years to reduce the residual PCB to a point where the final leach is reduced to an acceptable value (Gilbert Addis and Bentsu Ro, "Equilibrium Study of PCB's Between Transformer Oil and Transformer Solid Materials", EPRI PCB Seminar, Dec. 3, 1981).
The problem and its cause are discussed in L. A. Morgan and R. C. Ostoff, "Problems Associated with the Retrofilling of Askarel Transformers", IEEE Power Eng. Soc., Winter Meeting, N.Y., N.Y., Jan. 30-Feb. 4, 1977, pap. A77, p. 120-9. The solubility of a typical silicone oil in PCB is practically nil (&lt;0.5%) at temperatures up to and over 100.degree. C., while the solubility of PCB in the silicone ranges from only 10% at 25.degree. C. to 12% at 100.degree. C. While this limited solubility does not restrict the bulk silicone from dissolving the available free PCB, it does restrict the ability of the PCB to diffuse from the pores or interstices of the cellulosic matter.
Within any given pore filled with PCB-containing coolant, diffusion of PCB out must be accompanied by diffusion of silicone in. At some point within the pore there must exist an interface between the PCB-containing coolant and the silicone, across which neither material can very rapidly diffuse. Because the PCB is more soluble in the silicone than the reverse, the PCB will slowly diffuse into the silicone while the interface advances gradually into the pore. The limited solubility restricts the rate of diffusion and while this mechanism might eventually clean the pore of PCB, it is orders of magnitude slower than if the silicone and PCB were miscible. The high viscosity of the silicone (and many other coolants) is also an inhibiting factor. The result is a long drawn-out leach period of perhaps several years, during which the silicone must be continually filtered or periodically replaced to remove PCB's from it. Thus, the slow leaching of PCB's out of the solid insulation by the silicone is worse than no leaching at all since the dangers of a spill of PCB-containing materials will persist over a period of years Experimental studies by Morgan and Osthoff showed, for example, that effective PCB diffusivities into a typical silicone oil were only 1/10 of those into a 10 centistoke hydrocarbon oil. Although one might prefer, then, to retrofill with such a hydrocarbon oil, if it were not for the fire hazard of hydrocarbons, there still also is the problem of separating the PCB from the contaminated hydrocarbon oil which is high boiling like the PCB and like the silicone oil.
More importantly, undiluted PCBs are highly viscous and thus relatively immobile. Askarels contain PCB dissolved in "TCB" (trichlorobenzene) or mixtures of TCB and "TTCB" (tetrachlorobenzene) which thins out or reduces the viscosity of the PCB. TCB is much more soluble in silicone than is PCB and, therefore, TCB is removed from the askarel residing within the interstice of the insulation leaving highly viscous PCB (with or without small amounts of diluents, TCB or mixtures) within the interstices. Consequently, treatments with silicone (e.g. as in the Dow RetroSil system), without prior treatment according to this invention, are counter-productive and render the PCB remaining in th interstices even more difficult to remove. This can explain the lack of commercial success of prior systems in reclassifying transformers to a "non-PCB" status.